Mass flow measurement systems have used off-line techniques where the flow is accumulated over time, the fractions separated out and then weighed. Such techniques are slow and may not represent the actual conditions of real-time on-line feed systems. Herein mass flow and mass flow rate are used interchangeably.
Measurement of mass flow rate is relatively direct for one component systems. Herein "component" is defined as a gaseous, liquid or solid state of material. But, for mass flow rate measurement in multi-component systems, the fundamental problem is separating out the mass flow of a single desired component when the relative proportions of the multi-component mixture vary. This problem must be solved in a commercially effective technique.
As disclosed in the '453 patent, the velocity or flow rate of a slurry is measured by correlating a detectable signature of the flowing material upstream and downstream and with the time elased between the corralated signatures the velocity of flow can be determined. However, if the pattern changes between the two sensors, the correlation may be degraded and errors may occur. For thsi reason, the '453 patent places the sensor electrodes close to each other.
As discussed in the '184 patent, the volume fraction of each cmponent is measured indirectly where six electrodes, arranged around the flow pipe, are used to measure capacitance with the flowing components providing the dielectric material. The technique uses the fact that the different components each have different dielectric constants that affect the measured capacitance. But, a difficulty occurs due to the non-uniformity of th electric field distribution when the sensors are hemi-cylindrical, or part of cylinders. The '184 patent uses six circumferential electrodes placed around the insulated pipe section. The capacitance between a first electrode and the three opposing electrodes in measured; then the measurement is made between the electrode next to the first and the corresponding three opposing electrodes. This rotating measurement process is repeated until six measurements (covering all six electrodes) are made at high speed. The high speed is necessary to measure the capacitance of the contents of the pipe while only negligible flow occurs. One continuing limitation of prior art systems is that, although the electric field is rotated, there are still several zones of lower sensitivity within a cross section of the pipe. If, in a two component system, the components are non-uniformly distributed across high and low sensitivity zones, then some inaccuracy in the measured value may occur.
A limitation in mass flow measurement systems is the power line generated noise that reduces sensitivity in such systems.
Another limitation of such systems that utilize capacitive measurements is that the dielectric constants of the slurry materials will change with temperature and with pressure.
Accordingly it is an object of the present invention to incorporate correction factors for temperature and pressure into such volume fraction measurements.
It is an object of this invention to provide an improved electrode design to overcome the above illustrated limitation.
Another object of the present invention is to recognize power line generated noise and by manipulating the data substantially eliminate such noise from the stored signal data.
It is another object of the present invention to provide rapid on-line mass flow measurements suitable for use as a feedback or feedforward control parameter for controlling a processing system.
The above incorporated patents were advanced in the art for determining the volume fraction of the solid in a slurry ('184) and a solid-liquid slurry velocity ('453). An important object of the present invention, is to combine these two measured parameters with the known density of the catalyst to yield the mass per unit time or mass flow, in order to reduce waste while improving quailty of the resulting polyolefin product.